Method and apparatus for generating/evaluating in a picture signal encoding/decoding one or more prediction information items

ABSTRACT

Advanced Video Coding uses intra prediction for 4*4 pixel blocks whereby reconstructed samples from adjacent pixel blocks are used to predict a current block. Nine different intra prediction modes are available in AVC. In order to save bits for signalling the prediction modes, a flag and a 3-bit parameter are used. If this flag is set the most probable prediction mode, which is calculated from previous predictions, is used by the encoder and the decoder to reconstruct the actual prediction mode. If the flag is cleared, the 3-bit parameter is sent to select the prediction mode independently. According to the invention, the flag is applied more frequently, based on a prediction error threshold, instead of applying the optimum prediction mode for a current pixel block.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus forgenerating/evaluating in a picture signal encoding/decoding one or moreprediction information items, for example in an AVC encoder or decoder.

BACKGROUND OF THE INVENTION

According to the Advanced Video Coding (AVC) standard ISO/IEC 14496-10,or MPEG-4 part 10, or H.264, the encoding may select between inter andintra coding for pixel blocks in the pictures. Inter coding uses motionvectors for block-based inter prediction between different pictures.Intra coding uses spatial predictions within a picture to exploitspatial statistical dependencies in the source signal. Motion vectorsand intra prediction modes are specified for differing block sizes. Theprediction residual is further compressed using a transform to removespatial correlation inside the transform block before it is quantised.Finally, the motion vector information or the intra prediction modeinformation is combined with the quantised transform coefficientinformation and is encoded using either variable length codes orarithmetic coding.

A macroblock, consisting of a 16*16 block of luma samples and twocorresponding blocks of chroma samples, is used as the basic processingunit of the video decoding process. For inter prediction a macroblockcan be further partitioned down to blocks of 4*4 luma samples in size,whereby the selected size of inter prediction partitions is a trade-offbetween the coding gain provided by using motion compensation withsmaller blocks and the quantity of data needed to represent the data formotion compensation. Motion vectors are encoded differentially withrespect to predicted values formed from nearby encoded motion vectors.After inter prediction from previously decoded samples in other picturesor spatial-based prediction from previously decoded samples within thecurrent picture, the resulting prediction residual is split into 4*4blocks, which are converted into the transform domain where they arequantised.

The spatial intra-prediction for luminance pixel blocks (luma blocks) isbased on 4*4 blocks. Intra prediction can be carried out for I and SItype macroblocks. Inputs to this process are associated values ofreconstructed samples (prior to a deblocking filter operation) fromadjacent luma blocks of the corresponding neighbour macroblock or fromadjacent 4*4 luma blocks of the current macroblock. Outputs of thisprocess are 4*4 luma sample arrays that are part of the 16*16 luma arrayof prediction samples of the macroblock.

Nine different intra prediction modes are available in AVC encoding anddecoding: prediction in 8 different spatial directions and a modedenoted ‘Intra_(—)4×4_DC prediction mode’. However, intra predictionmodes for neighbouring 4*4 blocks are often correlated. In order to savebits for signalling the prediction mode to be used for a current 4*4block, a ‘prev_intra4×4_pred_mode’ flag and a 3-bit parameter‘rem_intra4×4_pred_mode’ are used. If this flag is set, the ‘mostprobable prediction mode’ is used by the encoder and the decoder. If theflag is ‘0’, rem_intra4×4_pred_mode is sent to indicate a change of mode(independent mode select). If rem_intra4×4_pred_mode is smaller than thecurrent ‘most probable prediction mode’ then the prediction mode is setto rem_intra4×4_pred_mode, otherwise the prediction mode is set to(rem_intra4×4_pred_mode+1). In this way, only eight values ofrem_intra4×4_pred_mode are required to signal the current one of thenine intra modes. The ‘most probable prediction diction mode’ means theminimum mode number of the prediction mode numbers for the 4*4 block tothe left of the current 4*4 block and for the 4*4 block top of thecurrent 4*4 block, respectively. If either of these neighbouring blocksis not available for prediction e.g. because it is not coded in Intra4×4mode, to the current 4*4 block the ‘Intra_(—)4×4_DC prediction mode’ isassigned.

SUMMARY OF THE INVENTION

The signalling of the rem_intra4×4_pred_mode for 4*4 luma blockpotentially results in an enlarged amount of signalling information bitsto be transmitted from the encoder to the decoders. However, suchsignalling information is side information and transmitting a largeamount of it reduces the available data rate for the ‘pure’ picture andaudio information (i.e. coded transform coefficients) and reducescorrespondingly the coding/de-coding picture or audio quality.

A problem to be solved by the invention is to improve, in a videoencoding or decoding offering multiple pixel block intra predictionmodes, the overall picture coding/decoding quality with respect to agiven average or maximum data rate by reducing (on average) thenecessary amount of side information to be transmitted or to beevaluated.

The invention solves this problem by means of more frequently applyingthe ‘most probable intra-prediction mode’ instead of applying the primafacie optimum intra-prediction mode for a specific picture content.

According to the above AVC standard, the video processing is based on16*16 pixel macroblocks each including in the 4:2:0 format four 8*8 lumablocks and two 8*8 chroma blocks, but an AVC encoder can perform anintra-prediction for luma sub-blocks of the size 4*4 pixels. Thereby foreach one of such 4*4 subblocks the nine different prediction modes aretested and the best match achieving the minimum prediction error, i.e.the minimum squared error, is selected as described above. According tothe invention, in order to further reduce the number of bits requiredfor transferring the required prediction mode information to thedecoder, both encoder and decoder use the ‘prev_intra4×4_pred_mode’ flageven in cases where the most probable prediction mode as defined in theAVC standard is not to be used. In other words, not the prediction modeaccording to the lowest possible prediction error is used, but theabove-described ‘most probable prediction mode’ is used even if it doesnot result in the best prediction match, i.e. the lowest possibleprediction error. Such inventive feature can be used for all lumasubblocks where the prediction error or miss for the most probableprediction mode is close to the best match.

The prediction error threshold, below which not therem_intra4×4_pred_mode information (three bits per item) is transmittedbut instead the ‘prev_intra4×4_pred_mode’ flag (1 bit) for a current 4*4luma block, is pre-defined or can be changed adaptively according to thecurrent type of picture content. Test with different picture materialhave shown that the penalty of the slightly increased prediction erroris compensated for by the reduction of the number of signalling bitswhich must be spent for the transmission of the actual prediction mode.Thereby advantageously the required bitrate for the encoded video signalis reduced, or for a give bitrate the coding/decoding quality isimproved.

In principle, the inventive method is suited for generating in a picturesignal encoding one or more prediction information items required in adecoding of the encoded picture signal for the prediction of a currentpixel block, said method including the steps:

-   -   either providing a first value for a prediction information item        said first value indicating to use for a current pixel block a        prediction mode that is to be derived from one or more        prediction modes of one or more previously predicted pixel        blocks that are adjacent to said current pixel block,    -   wherein said prediction information item first value has a first        word length and is for example a flag,    -   or providing a second value for said prediction information item        said second value indicating not to use for a current pixel        block a prediction mode that is to be derived from one or more        prediction modes of one or more previously predicted pixel        blocks that are adjacent to said current pixel block, and        providing the actual prediction mode value for said current        pixel block, said actual prediction mode being related to the        minimum prediction error for said current pixel block, wherein        said actual prediction mode value has a second word length that        is greater than said first word length, wherein said first value        for said prediction information item is provided and not an        actual prediction mode value for said current pixel block if,        when applying said first value for said prediction information        item instead of applying a minimum prediction error related one        of the available actual prediction modes, the actual prediction        error for said current pixel block is greater than when applying        said minimum prediction error related prediction mode but is        smaller or equal a predetermined prediction error threshold        value,    -   or is suited for intra predictinging in a picture signal        decoding a current pixel block by evaluating one or more        receieved prediction information items required for the        prediction of said current pixel block, said method including        the steps:    -   either evaluating a received first value for a prediction        information item said first value indicating to use for a        current pixel block a prediction mode that is to be derived from        one or more prediction modes of one or more previously predicted        pixel blocks that are adjacent to said current pixel block,        wherein said prediction information item first value has a first        word length and is for example a flag,    -   or evaluating a second received value for said prediction        information item said second value indicating not to use for a        current pixel block a prediction mode that is to be derived from        one or more prediction modes of one or more previously predicted        pixel blocks that are adjacent to said current pixel block, and        evaluating a received actual prediction mode value for said        current pixel block, said actual prediction mode being related        to the minimum prediction error for said current pixel block,        wherein said actual prediction mode value has a second word        length that is greater than said first word length,    -   wherein said received first value for said prediction        information item for an intra prediction of said current pixel        block is applied if, when applying said first value for said        prediction information item instead of applying a minimum        prediction error related one of the available actual prediction        modes, the actual intra prediction error for said current pixel        block would be greater than when applying said minimum        prediction error related prediction mode but is smaller or equal        a predetermined intra prediction error threshold value.

In principle the inventive apparatus is suited for generating in apicture signal encoding one or more prediction information itemsrequired in a decoding of the encoded picture signal for the predictionof a current pixel block, said apparatus including means being adaptedfor:

-   -   either providing a first value for a prediction information item        said first value indicating to use for a current pixel block a        prediction mode that is to be derived from one or more        prediction modes of one or more previously predicted pixel        blocks that are adjacent to said current pixel block, wherein        said prediction information item first value has a first word        length and is for example a flag,    -   or providing a second value for said prediction information item        said second value indicating not to use for a current pixel        block a prediction mode that is to be derived from one or more        prediction modes of one or more previously predicted pixel        blocks that are adjacent to said current pixel block, and        providing the actual prediction mode value for said current        pixel block, said actual prediction mode being related to the        minimum prediction error for said current pixel block, wherein        said actual prediction mode value has a second word length that        is greater than said first word length,    -   wherein said means are being adapted for providing said first        value for said prediction information item and not providing an        actual prediction mode value for said current pixel block if,        when applying said first value for said prediction information        item instead of applying a minimum prediction error related one        of the available actual prediction modes, the actual prediction        error for said current pixel block is greater than when applying        said minimum prediction error related prediction mode but is        smaller or equal a pre-determined prediction error threshold        value,    -   or is suited for intra predicting in a picture signal decoding a        current pixel block by evaluating one or more received        prediction information items required for the prediction of said        current pixel block, said apparatus including means being        adapted for:    -   either evaluating a received first value for a prediction        information item said first value indicating to use for a        current pixel block a prediction mode that is to be derived from        one or more prediction modes of one or more previously predicted        pixel blocks that are adjacent to said current pixel block,        wherein said prediction information item first value has a first        word length and is for example a flag,    -   or evaluating a second received value for said prediction        information item said second value indicating not to use for a        current pixel block a prediction mode that is to be derived from        one or more prediction modes of one or more previously predicted        pixel blocks that are adjacent to said current pixel block, and        evaluating a received actual prediction mode value for said        current pixel block, said actual prediction mode being related        to the minimum prediction error for said current pixel block,        wherein said actual prediction mode value has a second word        length that is greater than said first word length,    -   wherein said means are being adapted for applying said received        first value for said prediction information item for an intra        prediction of said current pixel block if, when applying said        first value for said prediction information item instead of        applying a minimum prediction error related one of the available        actual prediction modes, the actual intra prediction error for        said current pixel block would be greater than when applying        said minimum prediction error related prediction mode but is        smaller or equal a predetermined intra prediction error        threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are described with reference tothe accompanying drawings, which show in:

FIG. 1 Simplified block diagram of an AVC encoder;

FIG. 2 Simplified block diagram of an AVC decoder;

FIG. 3 Picture with a macroblock grid;

FIG. 4 Partitioning of macroblocks and sub-macroblocks;

FIG. 5 Available 4*4 intra prediction modes in AVC;

FIG. 6 Depicted direction of the prediction for different predictionmodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 the encoder video data input signal IE includes macroblockdata to be encoded. In case of intraframe data without prediction asubtractor SUB simply passes the data via transform means DCT, e.g.discrete cosine transform means, and quantising means Q to entropyencoding means ECOD which may perform variable length encoding orarithmetic coding, and which deliver via encoder buffer EBUF the encodervideo data output signal OE. In case of inter predicted data or intrapredicted data, subtractor SUB subtracts predicted macroblock data PMDfrom the input signal and passes the difference data via transform meansDCT and quantising means Q to entropy encoding means ECOD. The outputsignal of Q is also fed to inverse quantising means Q_(E) ⁻¹, the outputsignal of which passes through correspondingly inverse transform meansDCT_(E) ⁻¹ to adder ADDE in the form of reconstructed macroblock orsub-macroblock difference data RMDD. The output signal of ADDE isintermediately stored in frame store and motion estimation andcompensation means FS_MC_E which include a prediction error calculatorand which also perform motion compensation on reconstructed macroblockdata or reconstructed sub-macroblock data (4*4 luma block data) andwhich deliver predicted macroblock data or predicted 4*4 luma block dataPMD to SUB and to the other input of adder ADDE. In case of interprediction the data intermediately stored are data related to adifferent (e.g. the previous) picture whereas in case of intraprediction the data intermediately stored are previously processed datarelated to the current input picture. Quantiser Q, inverse quantiserQ_(E) ⁻¹, and entropy encoder ECOD are controlled basically by thefilling level of an output buffer BUF. The inventive intra predictionmode selection is carried out in motion estimation and compensationmeans FS_MC_E. The 4*4 intra prediction mode information PMI is added tothe data-stream OE to be transmitted e.g. in entropy encoder ECOD.

In FIG. 2 the decoder video data input signal ID passes through decoderbuffer DBUF and entropy decoding or arithmetic decoding means EDEC,inverse quantising means Q_(D) ⁻¹, and inverse transform computing meansDCT_(D) ⁻¹, e.g. inverse discrete cosine transform means, to an adderADDD which delivers the decoder video data output signal OD. The outputsignal of ADDD is fed to frame store and motion compensation meansFS_MC_D which also perform motion compensation on reconstructedmacroblock data or reconstructed sub-macroblock data (4*4 luma blockdata) and which deliver predicted macroblock data or predicted 4*4 lumablock data PMD to the other input of adder ADDE. The 4*4 intraprediction mode information PMI is regained from the received datastreamID, e.g. in stage EDEC, and is used to control stage FS_MC_Dcorrespondingly. In case of intraframe macroblock data withoutprediction the adder ADDD simply passes the output signal nal of DCT_(D)¹. In case of inter predicted data the data intermediately stored aredata related to a different (e.g. the previous) picture whereas in caseof intra predicted data the data intermediately stored are previouslyprocessed data related to the current input picture. Inverse quantiserQ_(D) ⁻¹ and entropy decoder EDEC are controlled according to the kindof control for Q_(E) ⁻¹ and ECOD. Normally, in motion compensation meansFS_MC_D the same motion vector information is used as in motioncompensation means FS_MC_E. The inventive intra prediction modeselection is carried out in motion compensation means FS_MC_D.

FIG. 3 shows that in AVC the pictures are composed of multiplemacroblocks, from which macroblock pairs MBP can be formed in theprocessing.

According to FIG. 4, in an AVC system the macroblocks or sub-macroblocksmay be partitioned, and the partitions are scanned for inter predictionor 4*4 intra prediction. The outer rectangles refer to the samples in amacroblock or sub-macroblock, respectively. The rectangles refer to thepartitions. The number in each rectangle specifies the index of theinverse macroblock partition scan or inverse sub-macroblock partitionscan. The upper part of FIG. 4 shows macroblock partitions MBT, the topright scheme showing four 8*8 sub-macroblocks per macroblock, whereasthe lower part shows sub-macroblock partitions SMBT of an 8*8sub-macro-block, the bottom right scheme showing four 4*4 luma samplesblocks per sub-macroblock. One of these blocks is the current block CBin the 4*4 intra processing. To the luma samples corresponding chromasamples and chroma sample blocks are associated.

In FIG. 5 the left column shows an AVC prediction mode index ‘0’ to ‘8’and the right column shows the corresponding 4*4 intra prediction modes.The corresponding prediction direction is depicted in FIG. 6, except formode ‘2’. The predictions are based on associated values ofreconstructed samples—prior to a deblocking filter operation—fromneighbouring macroblocks. For example, index ‘0’ denotes a verticalprediction from an adjacent 4*4 block of the adjacent upper macroblock,and index ‘1’ denotes a horizontal prediction from an adjacent 4*4 blockof the adjacent left macroblock.

Output for a 4*4 luma block are the prediction samples pred[x,y], withx, y=0 . . . 3, the position of the upper-left sample being assigned tox=O and y=O. There are 13 possible neighbouring reconstructed lumasamples p[x,y], with x=−1, y=−1 . . . 3 and x=0 . . . 7, y=−1, on whichthe intra prediction can be based, whereby each predicted luma block isprocessed and constructed into the current frame prior to decoding ofthe next luma block.

Mode ‘0’ (vertical prediction) is used only when the samples p[x,−1]with x=0 . . . 3 are available for the 4*4 intra prediction. The valuesof the prediction samples pred[x,y] are derived by pred[x,y]=p[x,−1],with x=0 . . . 3, y=0 . . . 3.

Mode ‘1’ (horizontal prediction) is used only when the samples p[−1,y],with y=0 . . . 3 are available for the 4*4 intra prediction. The valuesof the prediction samples are derived by pred[x,y]=p[−1,y], with x=0 . .. 3, y=0 . . . 3.

Mode ‘3’ (diagonal down left prediction) is used only when the samplesp[x,−1] with x=0 . . . 7 are available for the 4*4 intra prediction. Thevalues of the prediction samples are derived by (“>>2” denotes anarithmetic right shift of a two's complement integer representation ofthe related number by 2 binary digits): If x=3 and y=3, pred[x,y] =(p[6,−1] + 3*p[7,−1] + 2) >>2 ; Else pred[x,y] = (p[x+y,−1] +2*p[x+y+1,−1] + p[x+y+2,−1] + 2) >>2 .

Mode ‘4’ (diagonal down right prediction) is used only when the samplesp[x,−1] with x=0 . . . 3 and p[−1,y] with y=−1 . . . 3 are available for4*4 intra prediction. The values of the prediction samples are derivedby: If x>y, pred[x,y] = (p[x−y−2,−1] + 2*p[x−y−1,−1] + p[x−y,−1] +2) >>2 ; Else if x<y, pred[x,y] = (p[−1,y−x−2] + 2*p[−1,y−x−1] +p[−1,y−x] + 2) >> 2 ; Else pred[x,y] = (p[0,−1] + 2*p[−1,−1] + p[−1,0] +2) >> 2 .

Mode ‘5’ (vertical right prediction) is used only when the samplesp[x,−1] with x=0 . . . 3 and p[−1,y] with y=−1 . . . 3 are available for4*4 intra prediction. Let the variable V be set to V=2*x−y. The valuesof the prediction samples pred[x,y] are derived by: If V=0,2,4 or 6,pred[x,y] = (p[x−(y>>1)−1,−1] + p[x−(y>>1),−1] + 1) >> 1 ; If V=1,3 or5, pred[x,y] = (p[x−(y>>1)−2,−1] + 2*p[x−(y>>1)−1,−1] + p[x−(y>>1),−1] +2) >> 2 ; If V=−1, pred[x,y] = (p[−1,0] + 2*p[−1,−1] + p[0,−1] + 2) >> 2; If V=−2 or −3, pred[x,y] = (p[−1,y−1] + 2*p[−1,y−2] + p[−1,y−3] +2) >> 2 .

Mode ‘6’ (horizontal down prediction) is used only when the samplesp[x,−1] with x=0 . . . 3 and p[−1,y] with y=−1 . . . 3 are available for4*4 intra prediction. Let the variable V be set to V=2*y−x. The valuesof the prediction samples pred[x,y] are derived by: If V=0,2,4 or 6,pred[x,y] = (p[−1,y−(x>>1) − 1] + p[−1,y−(x>>1)] + 1) >> 1 ; Else ifV=1,3 or 5, pred[x,y] = (p[−1,y−(x>>1) − 2] + 2*p[−1,y−(x>>1) − 1] +p[−1,y−(x>>1)] + 2) >> 2 ; Else if V=−1, pred[x,y] = (p[−1,0] +2*p[−1,−1] + p[0,−1] + 2 ) >> 2 ; Else if V=−2 or −3), pred[x,y] =(p[x−1,−1] + 2*p[x−2,−1] + p[x−3,−1] + 2) >> 2 .

Mode ‘7’ (vertical left prediction) is used only when the samplesp[x,−1] with x=0 . . . 7 are available for 4*4 intra prediction. Thevalues of the prediction samples pred[x,y] are derived by: If y=0 or 2,pred[x,y] = (p[x+(y>>1),−1] + p[x+(y>>1)+1,−1] + 1) >> 1 ; Else if y=1or 3, pred[x,y] = (p[x+(y>>1),−1] + 2*p[x+(y>>1)+1,−1] +p[x+(y>>1)+2,−1] + 2) >> 2 .

Mode ‘8’ (horizontal up prediction) is used only when the samplesp[−1,y] with y=0 . . . 3 are available for 4*4 intra prediction. Let thevariable V be set to V=x+2*y. The values of the prediction samplespred[x,y] are derived by: If V=0,2 or 4, pred[x,y] = (p[−1,y+(x>>1)] +p[−1,y+(x>>1)+1] + 1) >> 1 ; Else if V=1 or 3, pred[x,y] =(p[−1,y+(x>>1)] + 2*p[−1,y+(x>>1)+1] + p[−1,y+(x>>1)+2] + 2) >> 2 ; Elseif V=5, pred[x,y] = (p[−1,2] + 3*p[−1,3] + 2) >> 2 ; Else if V>5,pred[x,y] = p[−1,3] .

In AVC intra_(—)4*4_DC prediction mode ‘2’ (DC prediction), if allsamples p[x,−1] with x=0 . . . 3 and p[−1,y] with y=0 . . . 3 areavailable for 4*4 intra prediction, the values of the prediction samplespred[x,y] are derived by:

-   -   pred[x,y]=(p[0,−1]+p[1,−1]+p[2,−1]+p[3,−1]+p[−1,0]+p[−1,1]+p[−1,2]+p[−1,3]+4)>>3;

Otherwise, if samples p[x,−1] with x=0 . . . 3 are not available for 4*4intra prediction and p[−1,y] with y=0 . . . 3 are available for 4*4intra prediction, the values of the prediction samples pred[x,y] arederived by:

-   -   pred[x,y]=(p[−1,0]+p[−1,1]+p[−1,2]+p[−1,3]+2)>>2;

Otherwise, if samples p[−1,y] with y=0 . . . 3 are not available for 4*4intra prediction and p[x,−1] with x=0 . . . 3 are available for 4*4intra prediction, the values of the prediction samples pred[x,y] arederived by:

-   -   pred[x,y]=(p[0,−1]+p[2,−1]+p[3,−1]+2)>>2;

If all samples p[x,−1] with x=0 . . . 3 and p[−1,y] with y=0 . . . 3 arenot available for 4*4 intra prediction, the values of the predictionsamples pred[x,y] are derived by:

-   -   pred[x,y]=128 , whereby a 4*4 luma block can always be predicted        using mode ‘2’.

The corresponding intra prediction for chroma samples is also used for Iand SI macroblock types only in AVC. Input are reconstructed chromasamples prior to the deblocking filter operation from neighbouringchroma blocks, if these are available. Output are intra predictionchroma samples for the current macroblock. Both chroma blocks (Cb andCr) of the current macroblock use the same prediction mode. Theprediction mode is applied to each of the chroma blocks separately. Moredetails are given in the above-mentioned AVC standard.

Opposite to existing video coding standards like MPEG-2, AVC facilitatesthe above-described intra-prediction of macroblocks. The luma componentfor each macroblock can be predicted based on a prediction of thecomplete 16*16 pixel macroblock or based on predictions for all sixteen4*4 pixel sub-blocks of a macroblock in nine different intra predictionmodes, i.e. per macroblock 9*16=144 4*4 intra prediction modeinformation items are required.

If all 4*4 blocks could be coded and decoded using the‘prev_intra4×4_pred_mode’ flags only, per macroblock 144 bits would berequired for transferring these flags. If all 4*4 blocks would be codedand decoded using the ‘rem_intra4×4 pred mode’ 3-bit information, permacroblock 144*3=432 bits would be required for transferring suchinformation. To reduce this potential source of increased bitrate, theAVC standard introduces the use of the above-described most probableprediction mode. Both, the encoder and the decoder calculate a mostprobable prediction mode for each intra predicted 4*4 pixel lumasubblock. The above-described inventive forced use of a flag or aminimum-bitlength information item indicating for a current small blockthe repeated application of a prediction mode derived from those usedfor one or more previous small blocks (leading to a non-minimumprediction error for the current small block) instead of applying anon-minimum-bitlength optimum mode information (leading to a minimumprediction error for the current small block) is applied if theprediction error for the current small block is up to about 10-15%greater than that prediction error for the current small block whichwould result if the optimum prediction mode would be applied. Tests haveshown that this leads to an advantageous overall bit rate reduction.

In a first embodiment, the most probable prediction mode is defined likein the current version of the AVC standard (FDIS ISO/IEC 14496-10) onthe above-described standardised prediction modes used for the two 4*4luma subblocks that are located above and to the left of the current 4*4sub-block.

In a second embodiment, the most probable prediction mode is not onlybased on the prediction modes of the small blocks (or subblocks)arranged to the left and on top of the current small block but also onthe small block located diagonally between those both small blocks, i.e.the top left neighbour small block.

If the increased prediction error for the current subblock is stillbelow (or equal to) the above-mentioned prediction error threshold, theencoder sets in stage FS_MC_E or in a non-depicted controller the flagindicating for the current small block the repeated application of aprevious prediction mode, but does not transmit the current predictionmode information (e.g. a three-bit information) for the current lumasubblock. In contrast, if the increased prediction error for the currentsubblock is above (or equal to) that prediction error threshold, theencoder clears in stage FS_MC_E or in a non-depicted controller the flagindicating for the current small block the repeated application of aprevious prediction mode and transmits prediction mode (or type)information to the decoder.

Depending on the information received, the decoder evaluates in stageFS_MC_D or in a non-depicted controller the flag(prev_intra4×4_pred_mode) indicating for the current small block therepeated application of a previous prediction mode and, as appropriate,calculates from the previously stored prediction modes of the twoadjacent sub-blocks (first embodiment) or from the previously storedprediction modes of the three adjacent sub-blocks (second embodiment)the prediction mode for the current sub-block and carries out acorresponding intra prediction, or carries out an intra prediction basedon the received prediction mode information (rem_intra4×4_pred_mode) forthe current small block.

“Most probable prediction mode” means in case of the second embodimentthat e.g. two of the three previously fixed prediction modes are thesame.

For all luma subblocks of a current frame, the encoder calculates thesub-block prediction error for all standardised prediction modes.Normally, the prediction mode resulting from the best (i.e. smallestpossible) prediction error would be used, but this approach results in avery high number of luma subblocks which are not predicted in the mostprobable mode. Instead, the smallest possible prediction error iscompared to the prediction error resulting from the use of the mostprobable prediction mode. If the prediction error difference isacceptable, i.e. is below a pre-defined or programmable threshold, themost probable prediction mode is used instead of the prediction modeyielding the lowest prediction error.

The invention can also be applied to a size of n*n of the small blocks,e.g. 8*8.

1. Method for generating in a picture signal encoding one or moreprediction information items required in a decoding of the encodedpicture signal for the prediction of a current pixel block, said methodincluding the steps: either providing a first value for a predictioninformation item said first value indicating to use for a current pixelblock a prediction mode that is to be derived from one or moreprediction modes of one or more previuosly predicted pixel blocks thatare adjacent to said current pixel block, wherein said predictioninformation item first value has a first word length and is for examplea flag, or providing a second value for said prediction information itemsaid second value indicating not to use for a current pixel block aprediction mode that is to be derived from one or more prediction modesof one or more previously predicted pixel blocks that are adjacent tosaid current pixel block, and providing the actual prediction mode valuefor said current pixel block, said actual prediction mode being relatedto the minimum prediction error for said current pixel block, whereinsaid actual prediction mode value has a second word length that isgreater than said first word length, wherein said first value isprovided for said prediction information item and not an actualprediction mode value for said current pixel block if, when applyingsaid first value for said prediction information item instead ofapplying a minimum prediction error related one of the available actualprediction modes, the actual prediction error for said current pixelblock is greater than when applying said minimum prediction errorrelated prediction mode but is smaller or equal a predeterminedprediction error threshold value.
 2. Method according to claim 1,wherein said encoding or said decoding is of type AVC and saidprediction is a 4*4 intra prediction for luminance and/or chrominanceblocks.
 3. Method according to claim 1, wherein said previouslypredicted pixel blocks that are adjacent to said current pixel block arethe blocks to the left and on top of the current pixel block, or are theblocks to the left, to the top left and on top of the current pixelblock.
 4. Method according to claim 1, wherein said threshold is changedadaptively according to the current type of picture content.
 5. Methodaccording to claim 1, wherein the level of said threshold is selectedsuch that the prediction error that is related to said threshold isabout 10-15% greater than the minimum possible prediction error thatarises when applying a corresponding one of the actually availableprediction modes.
 6. Method for intra predicting in a picture signaldecoding a current pixel block by evaluating one or more receivedprediction information items required for the prediction of said currentpixel block, said method including the steps: either evaluating areceived first value for a prediction information item said first valueindicating to use for a current pixel block a prediction mode that is tobe derived from one or more prediction modes of one or more previouslypredicted pixel blocks that are adjacent to said current pixel block,wherein said prediction information item first value has a first wordlength and is for example a flag, or evaluating a second received valuefor said prediction information item said second value indicating not touse for a current pixel block a prediction mode that is to be derivedfrom one or more prediction modes of one or more previously predictedpixel blocks that are adjacent to said current pixel block, andevaluating a received actual prediction mode value for said currentpixel block, said actual prediction mode being related to the minimumprediction error for said current pixel block, wherein said actualprediction mode value has a second word length that is greater than saidfirst word length, wherein said received first value is applied for saidprediction information item for an intra prediction of said currentpixel block if, when applying said first value for said predictioninformation item instead of applyling a minimum prediction error relatedone of the available actual prediction modes, the actual intraprediction error for said current pixel block would be greater than whenapplying said minimum prediction error related prediction mode but issmaller or equal a predetermined intra prediction error threshold value.7. Method according to claim 6, wherein said encoding or said decodingis of type AVC and said prediction is a 4*4 intra prediction forluminance and/or chrominance blocks.
 8. Method according to claim 6,wherein said previously predicted pixel blocks that are adjacent to saidcurrent pixel block are the blocks to the left and on top of the currentpixel block, or are the blocks to the left, to the top left and on topof the current pixel block.
 9. Method according to claim 6, wherein saidthreshold is changed adaptively according to the current type of picturecontent.
 10. Method according to claim 6, wherein the level of saidthreshold is selected such that the prediction error that is related tosaid threshold is about 10-15% greater than the minimum possibleprediction error that arises when applying a corresponding one of theactually available prediction modes.
 11. Apparatus for generating in apicture signal encoding one or more prediction information itemsrequired in a decoding of the encoded picture signal for the predictionof a current pixel block, said apparatus including means being adaptedfor: either providing a first value for a prediction information itemsaid first value indicating to use for a current pixel block aprediction mode that is to be derived from one or more prediction modesof one or more previously predicted pixel blocks that are adjacent tosaid current pixel block, wherein said prediction information item firstvalue has a first word length and is for example a flag, or providing asecond value for said prediction information item said second valueindicating not to use for a current pixel block a prediction mode thatis to be derived from one or more prediction modes of one or morepreviously predicted pixel blocks that are adjacent to said currentpixel block, and providing the actual prediction mode value for saidcurrent pixel block, said actual prediction mode being related to theminimum prediction error for said current pixel block, wherein saidactual prediction mode value has a second word length that is greaterthan said first word length, wherein said means are being adapted forproviding said first value for said prediction information item and notproviding an actual prediction mode value for said current pixel blockif, when applying said first value for said prediction information iteminstead of applying a minimum prediction error related one of theavailable actual prediction modes, the actual prediction error for saidcurrent pixel block is greater than when applying said minimumprediction error related prediction mode but is smaller or equal apredetermined prediction error threshold value.
 12. Apparatus accordingto claim 11, wherein said encoding or said decoding is of type AVC andsaid prediction is a 4*4 intra prediction for luminance and/orchrominance blocks.
 13. Apparatus according to claim 11, wherein saidpreviously predicted pixel blocks that are adjacent to said currentpixel block are the blocks to the left and on top of the current pixelblock, or are the blocks to the left, to the top left and on top of thecurrent pixel block.
 14. Apparatus according to claim 11, wherein saidthreshold is changed adaptively according to the current type of picturecontent.
 15. Apparatus according to claim 11, wherein the level of saidthreshold is selected such that the prediction error that is related tosaid threshold is about 10-15% greater than the minimum possibleprediction error that arises when applying a corresponding one of theactually available prediction modes.
 16. Apparatus for intra predictingin a picture signal decoding a current pixel block by evaluating one ormore received prediction information items required for the predictionof said current pixel block, said apparatus including means beingadapted for: either evaluating a received first value for a predictioninformation item said first value indicating to use for a current pixelblock a prediction mode that is to be derived from one or moreprediction modes of one or more previously predicted pixel blocks thatare adjacent to said current pixel block, wherein said predictioninformation item first value has a first word length and is for examplea flag, or evaluating a second received value for said predictioninformation item said second value indicating not to use for a currentpixel block a prediction mode that is to be derived from one or moreprediction modes of one or more previously predicted pixel blocks thatare adjacent to said current pixel block, and evaluating a receivedactual prediction mode value for said current pixel block, said actualprediction mode being related to the minimum prediction error for saidcurrent pixel block, wherein said actual prediction mode value has asecond word length that is greater than said first word length, whereinsaid means are being adapted for applying said received first value forsaid prediction information item for an intra prediction of said currentpixel block if, when applying said first value for said predictioninformation item instead of applying a minimum prediction error relatedone of the available actual prediction modes, the actual intraprediction error for said current pixel block would be greater than whenapplying said minimum prediction error related prediction mode but issmaller or equal a predetermined intra prediction error threshold value.17. Apparatus according to claims 16, wherein said encoding or saiddecoding is of type AVC and said prediction is a 4*4 intra predictionfor luminance and/or chrominance blocks.
 18. Apparatus according toclaims 16, wherein said previously predicted pixel blocks that areadjacent to said current pixel block are the blocks to the left and ontop of the current pixel block, or are the blocks to the left, to thetop left and on top of the current pixel block.
 19. Apparatus accordingto claim 16, wherein said threshold is changed adaptively according tothe current type of picture content.
 20. Apparatus according to claim16, wherein the level of said threshold is selected such that theprediction error that is related to said threshold is about 10-15%greater than the minimum possible prediction error that arises whenapplying a corresponding one of the actually available prediction modes.